Photomasks may be used to transfer photolithographic patterns onto a substrate during the manufacture of electronic components such as integrated circuits.
The fabrication of a photomask may involve a blanket deposition of an absorber material on a polished transparent substrate. The absorber material is then coated with a photoresist. The photoresist is exposed with a pattern generator. The pattern generator produces a beam, e.g. an electron beam, based on pattern information. This information may include data corresponding to the features to be printed on the photoresist. Following patterning, the photomask is developed to form the desired printed pattern in the photoresist. This pattern is then transferred into the absorber material by subjecting the photomask to chemical processing to remove material other than the desired pattern of absorber material from the substrate.
The pattern printed on a photomask includes a series of lines. The critical dimension (CD) of a photomask is related to the smallest width of a line or the smallest space between two lines permitted in the fabrication of a photomask. Plasma etching for reducing the CD of a photoresist line is termed photoresist trimming and may be used to reduce the CD. Similarly, plasma etching may be used to reduce the CD of a pattern associated with an absorber.
Differences in pattern densities effect CD uniformity during photomask fabrication. Etch loading effect pertains to a phenomenon occurring upon simultaneously etching a pattern of a higher density and a pattern of a lower density: due to a difference in etching rate of a material from one location to another, the amount of reaction products produced by etching becomes locally dense or sparse, and convection of a large amount of reaction products by etching causes a non-uniformity in etching rate. This etch-rate difference leads to CD variation between areas of high pattern density and low pattern density during the manufacture of photomasks.
Critical dimension non-uniformity attributable to etch loading effect has generally been addressed by performing an etching procedure under low pressure conditions. However, even at low pressure, CD non-uniformity of greater than 5 nm can remain. Proximity correction by the controlled application of an electronic beam (e-beam) may alleviate this CD non-uniformity. However, when pattern density variation becomes large this proximity correction technique can promote additional CD non-uniformity amounting to about 10 nm.